Beachcombers out on a stroll consider it a lucky omen when they find one. But then they aren’t found on many beaches. Many legends surround the intriguing common sand dollars (Echinarachnius parma). Some say they are coins lost by mermaids, or from the mythical Atlantis civilization.
They are also interpreted for their Christian symbolism. But the sand dollars are unique, burring sea urchins, closely related to sea lilies, starfish, sea urchins, and sea cucumbers.
These disc-shaped flat invertebrates are generally found in the coastal intertidal zone. The sand dollars are found in both temperate and tropical beaches, or the floors of the ocean in the Northern Hemisphere.
The Sand Dollars Change Color When They Die
They turn white and hard when they die. A living sand dollar can be a dark reddish-brown, green, blue, or even a shade of vibrant purple. They are enclosed in flexible spines (bristles) when alive that conceal the star design we are familiar with. Once they die, the test (skeleton) becomes hardened and the spines fall away.
They don’t live for long outside water. Collecting or handling living sand dollars is forbidden in maximum states. they can survive outside water only for a few minutes. They breathe through the petaloid, or petals, which are several holes that reveal the breathing tube.
They’re Related To The Sea Urchins And Sea Stars
The sand dollars are related to the sea urchins and the sea stars. They belong to the echinoids class of spiny-skinned beings. They share a lot of the anatomy of sea urchins and sea cucumbers.
Another common name used in the United States for the sand dollars is ‘cake urchin,’ sea biscuit,’ and ‘eccentric sand dollar.’ In New Zealand, ‘snapper biscuit’ and ‘sea cookie,’ are some names used. The flower-like shell pattern has led it to be named ‘pansy shell’ in South Africa.
The Spines Are Used For Feeding
The sand dollars use the spines to consume. The critters survive on small copepods, crustacean larvae, diatoms, microscopic algae, and debris. Their spines are covered in flexible and tiny bristles named cilia. The sand dollars use these to shift food particles from the sand, and along with that prickly body surface into their month, located centrally at the bottom.
The sand dollar keeps the crab larvae and amphipods in a small, teepee-shaped funnel of spines before feeding on them. The sand dollars generally feed on Zooplankton, phytoplankton, and detritus, which are organic matter produced by the decomposition of organisms.
The mouth of the sand dollar has a jaw that has 5 segments for grinding its food for around 15 minutes, and then it swallows them. It takes a couple of days for it to digest its food.
The distinctive flower design on the dead sand dollars are five sets of water and gas processing pores arranges in an attractive symmetrical pattern. They have an important purpose for the living echinoid. Called lunules, they are pressure drainage channels that prevent the sand dollar from getting carried away by the ocean currents. They are also useful for harvesting food.
The sand dollar stands upright in the sand with one end when oceans are calm. In strong currents, the sand dollars lie flat and burrow beneath the sand, and stay anchored in one place. They also use other methods like growing thicker and heavier skeletons or ingesting sand to make them heavier and weigh them to the ocean floor.
They Love A Crowded Lifestyle That Boosts Reproduction
The sand dollars stay together in large numbers and are not choosy about the living area. A single square yard (0.8 of a square meter) may contain as many as 625 sand dollars. It is probably linked to their reproductive cycle.
The sand dollar reproduces through group spawning or ‘broadcast.’ Both males and females release sperm and eggs in the water in that enclosed space. The more concentrated they are, the greater are the chances of reproductive success. The sand dollars find the intertidal zones easier for reproduction.
Once the gametes are released into the water they are conceived by the female sand dollar. This process of external fertilization makes them ‘broadcast’ spawners. Thus, when one of them begins spawning, the others join in the spawning process. This multiplies the prospect of their reproduction, fertilization, and boosts the endurance of the species. The average litter size can be as high as 350,000.
Very Few Predators Ensures They Survive Long
The sand dollars are lucky to have very few predators. their hard exoskeleton and lack of edible parts ensure that they don’t have a lot of predators. only the ocean pout, which is wide fleshy-mouthed fish similar to the eel, the California sheep heads, the pink sea star, and the starry flounder considers the sand dollar as prey. They eat the sand dollar larvae. That makes the sand dollar a prey of its kind.
The rings of the sand dollars can help us determine its age. It is somewhat akin to the rings of a tree that help us count the years a tree has lived. Their sand dollars sport growth rings on their test. With age, the quantity of rings grows. So the older the creature gets, the bigger it is.
The average weight of a sand dollar is less than 30 grams (1.06 ounces) and they can grow to a length between 2 and 4 inches (5 and 10 centimeters). The disk-like ocean dwellers can survive anywhere between six and ten years. The creature dies when they face a lack of food in their surrounding waters.
Humans constitute the single largest threat to the sand dollars. As they have very few predators, they live longer than many sea creatures their size.
Image credits: Getty Images